1. Field of the Invention
The present invention relates to an internal combustion engine system controller for controlling an internal combustion engine system including an internal combustion engine having a plurality of cylinders (hereinafter, referred to as “multi-cylinder internal combustion engine”).
2. Description of the Related Art
Conventionally, a device for controlling an air-fuel ratio based on output signals from an upstream-side air-fuel ratio sensor (hereinafter, referred to as “A/F sensor”) and a downstream-side air-fuel ratio sensor (hereinafter, referred to as “O2 sensor”) respectively provided in an exhaust passage on an upstream side and a downstream side of an exhaust-gas purification catalyst is known (for example, see Japanese Patent Application Laid-open Nos. 2004-183585 and 2006-104978 and the like). The A/F sensor is an oxygen sensor exhibiting linear output characteristics with respect to the air-fuel ratio. On the other hand, the O2 sensor is an oxygen sensor exhibiting so-called Z-characteristics (characteristics in which an output changes in a stepwise manner in a mode in which an output abruptly changes between a rich side and a lean side with respect to a stoichiometric air-fuel ratio as a reference) with respect to the air-fuel ratio.
In the above-mentioned type of device, a fuel injection amount is subjected to feedback control so that the air-fuel ratio of an exhaust gas flowing into the exhaust-gas purification catalyst (hereinafter, referred to simply as “catalyst”) becomes equal to a target air-fuel ratio based on the output signal from the A/F sensor (hereinafter, the control is referred to as “main feedback control”). With the main feedback control, control for feeding-back the output signal from the O2 sensor to the fuel injection amount is also performed (hereinafter, the control is referred to as “sub-feedback control”).
More specifically, in the main feedback control, a feedback correction amount is calculated based on a deviation between the air-fuel ratio of the exhaust gas, corresponding to the output from the A/F sensor, and the target air-fuel ratio. On the other hand, in the sub-feedback control, a sub-feedback amount (sub-feedback correction amount) is calculated based on the output signal from the O2 sensor. Then, the sub-feedback amount is fed-back to the main feedback control to correct the deviation between the air-fuel ratio of the exhaust gas, corresponding to the output from the A/F sensor, and the target air-fuel ratio.
In the sub-feedback control, a steady component contained in the sub-feedback amount is acquired as a sub-feedback learning value. The sub-feedback learning value corresponds to a constant error contained in the output signal from the A/F sensor. Therefore, by the feedback of the sub-feedback learning value to the main feedback control, the above-mentioned constant error is compensated for. As a result, an actual air-fuel ratio can be made closer to the target air-fuel ratio quickly after the start of correction of the air-fuel ratio by the sub-feedback control.
The above-mentioned sub-feedback control learning value is also used to determine whether or not a deviation in actual air-fuel ratio among a plurality of cylinders has become excessive and to determine the occurrence of an accidental fire (for example, see Japanese Patent Application Laid-open No. 2009-30455 and the like; the details of the determinations of occurrence of an abnormality described above are described with the description of an embodiment of the present invention).
As described above, the sub-feedback learning value is extremely useful and important in various usages of the above-mentioned type of device. Therefore, various configurations have been conventionally proposed so that the sub-feedback learning value quickly converges with high accuracy (for example, see Japanese Patent Application Laid-open Nos. 2009-162139 and 2009-180145 and the like).